1. Technical Field
The present disclosure relates to a constant voltage circuit including an excess-current protection circuit to protect against excess current by alternately decreasing output voltage and output current in stages, and an electronic device including the constant voltage circuit.
2. Description of the Related Art
FIG. 1 is a circuit diagram illustrating a known constant-voltage circuit 100-X that changes an output voltage and an output current in stages. The constant-voltage circuit 100-X includes, in addition to an excess-current protection circuit 10e, a reference voltage generator circuit 1 to generate a reference voltage Vref, an error amplifier 2, an output MOS transistor M1, and an output detection circuit 3 including a variable resistor R21 and a fixed resistor R22. The excess-current protection circuit 10e includes MOS transistors M112, M113, M114, M151, and M152 and resistors R123, R124, R125 and R29. The configuration of the constant-voltage circuit 100-X is typical and thus a description thereof is omitted. The operation of the excess-current protection circuit 10e is described below.
FIG. 2 is a graph illustrating the characteristics of an output voltage Vout relative to an output current Iout of the constant-voltage circuit 100-X.
In the constant-voltage circuit 100-X of FIGS. 1 and 2, a source and a gate of the MOS transistor M112 are connected to a source and agate of the MOS transistor M1, and a drain current of the output MOS transistor M112 is proportional to a drain current of the MOS transistor M1. Then, the drain current of the MOS transistor M112 flows through the resistor R123, which generates a voltage across the resistor R123.
When the voltage across the resistor R123 reaches a threshold voltage of the MOS transistor M113, the MOS transistor M113 is turned on. The drain current of the MOS transistor M113 generates a voltage across the resistor R29 to switch the MOS transistor M114 on.
Herein, since the drain of the MOS transistor M114 is connected to the gate of the output MOS transistor M1, the MOS transistor M114 is switched on, which acts to increase gate voltage of the output MOS transistor M1. Accordingly, an increase in the output current Iout of the output M1 is suppressed, and then the output voltage Vout starts declining. The output current Iout at this time is a first limited current IL1.
The MOS transistor M151 is set to be on while the output voltage Vout is at or over a predetermined voltage. When an excess current flows and the output voltage Vout declines to a first limited voltage VL1 through the above-described process, a junction voltage VFB between the resistors R21 and R22 of the output voltage detection circuit 3 is decreased, which decreases the gate voltage of the MOS transistor M151. When a gate voltage of the MOS transistor M151 is decreased to the predetermined voltage, the MOS transistor M151 is switched off, and the drain current of the MOS transistor M112 flows through not only the resistor R123 but also the resistor R124. Accordingly, a gate voltage of the MOS transistor M113 is increased, which increases the gate voltage of the output MOS transistor M1 via the MOS transistors M113 and M114, and decreases the output current Iout of the constant-voltage circuit 100-X from the first limited current IL1 to a second limited current IL2.
As the output voltage Vout is decreased to a second limited voltage VL2 through the foregoing process, the MOS transistor M152 is switched off, and the drain current of the MOS transistor M112 flows not only to the resistor R125 but also to the resistors R123 and R124. Accordingly, the gate voltage of the MOS transistor M113 is increased, which further increases the gate voltage of the output MOS transistor M1 via the MOS transistors M113 and M114, and further decreases the output current Iout of the constant-voltage circuit 100-X from the second limited current IL2 to a third limited current IL3.
Accordingly, the constant-voltage circuit 100-X shown in FIG. 1 changes the output voltage Vout and the output current Iout in stages, as shown in FIG. 2.
In a constant-voltage circuit configured as described above, a package of the power supply integrated circuit (IC) is compact and power dissipation is not great. Therefore, when the excess current flows through the constant-voltage circuit 100-X, heat generation is prevented using the excess-current protection circuit that alternately changes the output voltage and the output current in stages and prevents delay in rising speed.
However, when a connected load fluctuates significantly, the undershoot of the output voltage is great. As a result, the output voltage Vout is trapped at a first step (e.g., first limited voltage VL1) of the excess-current protection circuit 10e, which may generate the failure that the output voltage Vout is not recovered from the trapped step. In particular, when the output voltage Vout is set at a low value, a voltage difference between an output setting voltage Vset and the first step voltage in stages is smaller, the non-recover failure is more likely to occur.